The president was referring to a statement by Mr Khan earlier this morning which said he was "grieving" for the victims and insisted that terrorists "would not win."

"Londoners will see an increased police presence today and over the course of the next few days. There's no reason to be alarmed," Mr Khan said.

He went on to condemn the attack in the strongest possible terms and ask for people to remain vigilant.

Mr Trump's comments were instantly ridiculed on social media.

Police forensics investigators work on London Bridge and Borough Market

"Cheap, nasty and unbecoming of a national leader," said the Labour parliamentary candidate David Lammy.

Some Conservative politicians including Penny Mordaunt stood up for the Labour Mayor.

A spokesman for Mr Khan said: "The mayor is busy working with the police, emergency services and the Government to co-ordinate the response to this horrific and cowardly terrorist attack and provide leadership and reassurance to Londoners and visitors to our city.

"He has more important things to do than respond to Donald Trump's ill-informed tweet that deliberately takes out of context his remarks urging Londoners not to be alarmed when they saw more police - including armed officers - on the streets."

I am also guilty of tweeting approval of a Labour Mayor's excellent statement on behalf of Londoners https://t.co/SIfWurebOb